Can RAID 1 (mirror) improve average access time?

Although they say RAID does not improve access time, supposedly, RAID 1 is able to improve average access time (but not random access time) during the read.

As you know there are two same copies of data and two member disks in RAID 1.

 When read operations are grouped as described below:

• the first half of the data is served by the first disk
• second disk is responsible for the second part of the data

then the average length which a read head should go to find a sector is reduced in half.
Based on the above considerations it is not possible to improve rotational latency. Anyway, performance improves because less head travel is needed. As for writes, it is not possible to improve performance since both disks should be updated.

Why are only the image thumbnails recovered?

More often than not when recovering digital images, typically off a memory card, the image thumbnails are extracted OK, while high-resolution images themselves are not.
There is a phenomenon called "file fragmentation", when the file is placed on the disk in several non-contiguous parts. The sketch showing the process can be seen at the Photo Recovery Limitations page

As a rule memory cards are formatted to FAT or FAT32. If the file gets erased on FAT, only the information how to locate its first fragment remains available. The second and the following fragments cannot be located.
Thumbnail is not big and it is placed near to the start of the file, therefore it can be often recovered successfully, whilst the full-size image is lost. Of all the recovery types, this limitation mostly applies to digital image recovery and unformat on FAT.